1. Field of the Invention
The present invention relates to a method and apparatus for detecting a short circuit point between wiring patterns of an LSI, and more particularly, a method and apparatus for detecting a short circuit point between wiring patterns using a non-contact detection method in a test process of a manufactured LSI.
2. Description of Related Art
In a test process of the manufacturing processes of an LSI, it is difficult to detect a short circuit point. Specifically, in a case of completion of an LSI circuit such as an application specific IC (ASIC) ordered by a user for delivery within a short period of time, a short circuit problem sometimes occurs. A short circuit is typically formed between a plurality of power supply wiring patterns because there are crosses between power supply wiring patterns and because bugs of wiring design data, error in setting the coordinates of wiring patterns, or overlapping wiring patterns are occasionally introduced in the CAD process. Such a type of short circuit between the power supply wiring patterns is the cause of incomplete operation of logic circuits as a main functional circuit of the ASIC.
In highly integrated circuits, it is very difficult to detect a point where the short circuit is formed. Conventionally, there is a method of visually checking the wiring mask pattern as a method of detecting a short circuit point between wiring patterns. However, it is impossible to completely check the wiring mask pattern for a large scaled integrated circuit layout.
There are two conventional methods of physically detecting a short circuit point between wiring patterns other than the visual method, a first method being a method of detecting a heat generating point using liquid crystal and a second method being a method of burning off a short circuit point by flowing high current.
The first conventional method of detecting a short circuit point between wiring patterns is described in "Very Sensitive Detection for LSI's Hot Spot using Liquid Crystal" (Proceedings of 6th International Conference on Reliability and Maintainability (Strasbourg France) October, 1988, pp. 516 521-521) (reference 1). According to reference 1, the temperature at a short circuit point between two power supply wiring patterns, e.g., a 5 V power supply line (VDD) and a ground line (GND) is increased as the voltage of power supply line VDD is increased. When temperature of liquid crystal is increased so that a phase transition temperature (Tc) of the liquid crystal is exceeded, the molecule structure of liquid crystal changes from a liquid crystal structure to a liquid structure. At the same time, the optical characteristics change from a birefringence state to an isotropic state. The change can be observed by a polarization microscope. Therefore, a thin liquid crystal film is coated uniformly on an LSI for the short circuit point detection and then power is supplied. In this case, if there is no short circuit point, the entire LSI does not exceed the phase transit temperature and the phase transition of liquid crystal does not occur. If there is any short circuit point, temperature of the short circuit point is increased and the phase transition of liquid crystal is caused. Therefore, when the LSI is observed by the polarization microscope, the presence or absence of a short circuit point can be observed. A liquid crystal having a liquid crystal characteristic at room temperature and having a phase transition temperature at 35.degree. C. or above which is somewhat higher than the room temperature is selected, in order to make the testing current as small as possible. Further, the entire LSI is heated slightly below the phase transition temperature (Tc) using a temperature controller to detect small heating up, as shown in FIG. 1A. As a result, a small increment of the temperature enables the liquid crystal to reach the phase transition temperature. Alternatively, the entire LSI can be heated slightly over the phase transition temperature (Tc), as shown in FIG. 1B. In this case, when the entire LSI is gradually cooled, a small heating value point can be observed when the temperature of LSI crosses the phase transition temperature (Tc). However, even in this method, it is essential to increase current for detecting a short circuit point between power supply wiring patterns.
Power consumption is expressed as P=V.multidot.I=R.multidot.I.sup.2 as is well known. It is necessary to increase the power consumption in order to increase the heating value. However, since impedance R is extremely small at the short circuit point between wiring patterns, current I needs to be increased to increase power consumption.
The second conventional method of detecting a short circuit point between wiring patterns is a method where a large current is supplied to power supply wiring patterns of an LSI to increase a current density at a short circuit point. Utilizing that current concentrated at the short circuit point, the short circuit point is fused. In order to fuse the short circuit point between the power supply wiring patterns, electron migration needs to be caused so that the short circuit point gets higher current density than other portions of power supply wiring patterns. In the electron migration phenomenon, cavities are generated on the current supply side of the wiring patterns through moment exchange between electrons as carriers of current and metal lattices, and protrusion or needle-like crystal growth is caused on the opposite side.
Recently, an application specific IC (ASIC) such as a gate array has been widely used. It includes standardized functional circuits such as standard cells and gate arrays and the wiring design is made to a semiconductor substrate somewhat standardized using a computer-aided design (CAD). After the wiring design is complete, a mask for wiring patterns is produced and applied to the semiconductor substrate in which a diffusion process is completed, to form the wiring patterns, resulting in circuits having predetermined functions. Specifically, in a case of ASIC, since the power supply wiring patterns for an internal logic circuit are formed in a network manner, current density is extremely decreased other than at the short circuit point. For this reason, the short circuit point is first fused through application of large current. However, there is a possibility that a semiconductor element having no relation to the short circuit point is damaged because application of a high voltage is necessary for application of a large current. In order to prevent damage to the semiconductor element, the current value must be limited to some extent and therefore it is difficult to precisely specify the short circuit point.
In the above mentioned first and second conventional methods and apparatus for detecting a short circuit point between wiring patterns, there is a drawback in that increase in temperature through application of a large current is necessary for detection of a short circuit point.
Further, in the conventional second method of detecting a short circuit point between wiring patterns, there is a drawback in that it is difficult to precisely specify a short circuit point because the current value is limited in order to prevent a semiconductor element from being damaged through application of a high voltage accompanied with the application of large current.